1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus.
2. Description of the Related Art
In recent years, high image quality, speed-up, and cost reduction are increasingly demanded of electrophotographic image forming apparatuses, such as laser printers and copiers. As for the high image quality in particular, stable high-quality image formation is required even when environment such as temperature and humidity changes. Technologies for providing an optical element with a variety of functions have been proposed to meet these demands. One of the technologies uses diffractive optical effect. Such a technology using diffractive optical effect is explained below.
A significant matter in properties of an optical scanning device about the stability of image quality is a change of a beam waist position (change of a focus position) due to temperature change. When the device is placed in environment which changes and if the temperature actually changes, this temperature change causes an optical element to thermally expand and a refractive index thereof to change. The position of the beam waist thereby deviates from a surface to be scanned, and the size of a light spot on the surface changes. If the size of the light spot increases or varies, the resolution of an image is reduced and image quality is degraded due to, for example, degradation of graininess, which interferes with stable image formation.
Hence, an optical system has been proposed in which an optical element with diffraction effect performs part of imaging function. The change in the temperature causes the refractive index of the optical element or a lens particularly made of resin such as plastic to change, and causes the lens to thermally expand, which leads to change of the beam waist position. At the same time, an oscillation wavelength of a light source varies. A focal length of a diffractive optical element is caused to vary due to the wavelength variation. It is known that the direction of the variation is opposite to that of the change in the focal length of the lens.
As a technology using this property, for example, Japanese Patent No. 3397638 discloses an optical system in which a scanning optical system includes a diffraction portion and a refraction portion. More specifically, in the optical system, by defining respective powers of the both portions, the change of the focus position which is variation of aberration is canceled out and corrected by a change in power of the refraction portion due to temperature change and by a change in power of the diffraction portion when the wavelength of the light source changes due to the temperature change. Thus, the change of the focus position in a sub-scanning direction can be prevented.
Japanese Patent Application Laid-Open No. 2002-287062 discloses a laser scanning device in which a light-source optical system collects light beams emitted from a light source near a deflecting surface in a sub-scanning direction. More specifically, the light-source optical system includes one optical element having a diffractive optical surface and a reflecting surface without a rotational symmetric axis.
As another method of using the diffraction effect, there is a method provided for deflection in addition to the method of causing the optical element to have the imaging function. For example, Japanese Patent Application Laid-Open No. 2003-241130 discloses an optical scanning device that diffracts light beams using a liquid-crystal deflecting element, deflects it in a sub-scanning direction and/or a main scanning direction, and correctively adjusts the position of a light spot. This optical scanning device including a ghost-light removal unit is proposed. The ghost-light removal unit blocks ghost light having occurred, to solve the problem in which there occurs another order of diffracted light being possibly the ghost light, other than normal diffracted light.
In the conventional technologies, however, when the diffractive optical element is used, a problem on decrease in diffraction efficiency which does not occur in a refractive optical element is not solved. As for the diffractive optical element, when the wavelength of the light source varies, the focal length changes, which also causes the diffraction efficiency of the diffractive optical element highly sensitive to the wavelength, to change. The diffraction efficiency or light-use efficiency decreases by deviation of wavelength from design wavelength of the element, and the image quality deteriorates caused by the change of light amount. As described in Japanese Patent Application Laid-Open No. 2003-241130, another order of diffracted light becomes stray light which is not used for desired optical scanning, and when this diffracted light occurs and reaches a photosensitive member, the image quality is degraded. To prevent the degradation, a shielding element or the like needs to be newly provided, which causes an increase in the number of components. This increase causes cost to increase and influence to be exerted on layout of the device, which results in upsizing of the device.